Mobile users to carriers: ‘Give me my unlimited data’

FORTUNE — While number one and two mobile carriers Verizon Wireless and AT&T have adopted tiered plans for wireless data, T-Mobile and Sprint have opted to stick with so-called “all-you-can-eat” options for customers. And unlike older so-called “unlimited” plans, which tended to be anything but truly “unlimited,” T-Mobile claims its new plan won’t throttle down speeds or place any caps on data usage.

This is just the latest move from the major mobile carriers, with each looking to draw customers with promises of either shared data or unlimited data, not to mention various options for unlimited talk time and texting. But are the carriers actually asking their respective consumers what they want, or does that even matter?

Overall, consumers seem less than enthused when it comes to shared data plans (when several different devices all eat from the same data trough, so to speak). According to a recent Parks and Associates study, 40% of mobile phone users would actually rather switch mobile carriers than use a shared mobile data plan. Some 38% of respondents in the survey suggested that they would voluntarily adopt a shared data plan but not switch providers for it. And just 10% said they’d choose a shared plan if it was the only way of sticking with their current provider.

By contrast, the Parks Associate study found that some 31% of AT&T T subscribers, 30% of Verizon vz users, 35% of T-Mobile subscribers, and 44% of Sprint Nextel s customers actually prefer unlimited data plans, even if they cost more.

“This move by T-Mobile to offer an unlimited plan is a good thing for the American mobile market,” says Ian Fogg, senior principal analyst and head of the IHS mobile sector at IHS iSuppli. “It is very good for a competitive marketplace.”

But it could mean those used to one carrier might have to make the switch to get what they want — or, rather, what they think they want — as even the carriers aren’t always completely clear on what they are offering. Currently, Sprint, T Mobile, and C Spire all offer unlimited data plans. “T Mobile is jumping into this unlimited space because they are losing business trying to compete against AT&T and Verizon,” says telecommunications analyst Jeff Kagan of Jeff Kagan and Associates. “T-Mobile does not have many fast 4G cities, but they are hoping to make progress against Sprint and C Spire.”

T-Mobile has promised to offer a 4G unlimited data plan that has no cap and no throttling, unlike its previous plan, which did slow down a user’s speed when they reached a certain data use threshold. The new plan, which includes unlimited voice, text, and data, will cost $90 for a single T-Mobile user with a subsidized phone. “It is for the data hungry user, so they can get as much as they want,” says Harry Thomas, director of segment marketing at T-Mobile. “Our unlimited nationwide 4G plan is about offering worry free options for consumers.”

The fact that AT&T and Verizon have moved away from unlimited plans while T-Mobile is supporting them suggests that carriers are still adapting to a changing market and usage patterns as consumers increasingly adopt smartphones and other portable devices. “The carriers are still learning to price the data,” Fogg says. “Everyone is still trying to work out what business model, what structure is the right one, especially as the number of devices that consumers can connect to is expanding. The question is, ‘What is the right way to do business?’”

Those consumers who are grandfathered in to unlimited plans may see service slow down. And those using the old 3G unlimited plans could see slower data speeds when they connect to heavily used mobile towers.

A service slowdown on other carriers could benefit T-Mobile, but the company is looking for additional ways to lure customers. While the nation’s fourth-largest national mobile carrier is the only one of the big four not to actually sell Apple’s aapl iPhone, it recently launched a campaign calling for iPhone users to bring their device to the carrier. The so-called “Unlocked & Unlimited” campaign would allow those with unlocked iPhone handsets to sign up for a new plan with T Mobile. The key is that the device must be unlocked, and it further suggests that T-Mobile will not be selling the iPhone in the near future. However, it’s estimated that there are more than a million iPhones currently running on T-Mobile’s network.

“Will T Mobile attract users of other carriers with their unlimited plan? Yes,” says Kagan. “Sprint Nextel is doing to the same thing. The marketplace is splitting into two parts. Customers just have to choose the part that fits them best. Do they want the fastest network? AT&T and Verizon. Do they want the unlimited feature? Sprint Nextel, C Spire Wireless and now T-Mobile.”